1. Field of the invention
The present invention relates to variable reluctance machines and, more particularly, to a high-output variable reluctance motor incorporating two or more rotating permanent magnets which alter the magnetic structure of the motor to magnetize the machine in the conventional manner and also to improve the flux linkage variation of each phase for more amenable operation with a power electronic converter.
2. Description of the Background
In a variable reluctance motor (VRM), torque is generated by a doubly-salient rotor and stator which tend to align themselves in order to reach a position of minimum magnetic reluctance. Under ideal switching conditions, VRMs have the potential to be highly efficient. Moreover, VRMs can be constructed with shorter end-windings, and they use less end-space. Hence, the length of the rotor and stator can be increased without adding to the overall motor dimensions or manufacturing costs. In this manner, the torque output can be increased substantially over a comparably-sized synchronous or induction motor.
Thus far, the VRM has fallen short of its above-described potential for two reasons. First of all, like all single excited motors, the VRM suffers from an "excitation penalty," i.e., the armature current must provide an excitation component, and thus has a lower efficiency and lower power factor (or energy ratio) as compared with motors wherein the sole purpose of the stator current is torque production. Secondly, ideal switching conditions do not exist, and attempts to achieve such conditions have resulted in commercially impractical converter circuits with excessive voltage and current stresses on the switching devices. In a variable reluctance motor, the current in each phase should be decreased to zero immediately when the rotor is aligned with that phase. The problem lies in a large turn-off inductance. It is difficult to accomplish this instantaneous current reduction because the phase inductance is a maximum when the rotor is aligned with that phase. Hence, the excitation current can be removed, but the inductance will draw a residual current from the energy stored in the magnetic field. The decaying residual current induces a detrimental reverse-torque as the rotor pole surpasses alignment with the corresponding stator pole. The problem is most serious when the speed of the motor is high.
As a result, those skilled in the art view present VRM technology with some skepticism. Synchronous or induction motors remain the industry choice.
It would be greatly advantageous to realize the full potential of a VRM by solving the above-described problems without increasing the complexity and cost of the converter or the motor.